This invention relates to ultrasonic nondestructive testing of workpieces, and more particularly for converting analog utrasonic echo signals arising from ultrasonic search signals intercepting an acoustic discontinuity in a workpiece into digital signals which may be stored.
In pulse-echo ultrasonic testing of materials an electro-acoustic transducer, for example a piezoelectric transducer, acoustically coupled to a workpiece transmits ultrasonic search signals into such workpiece. When the search signal intercepts an acoustic discontinuity, such as the entrant or rear will surface of the workpiece or an acoustic discontinuity disposed in the workpiece, a portion of the signal is reflected back toward the transducer. The reflected ultrasonic signals from the workpiece, which commonly are referred to as ultrasonic echo signals, are received by the electroacoustic transducer and transformed into electrical signals. By comparing the amplitudes of two or more successive echo signals and the time differences between these signals, or by comparing the echo signals with signals received from a reference defect, data is provided for analyzing the workpiece under test. A common method of displaying or illustrating the instantaneous values of the echo signals is to provide an A-scan representation on the screen of a cathode ray tube, which display is an amplitude versus time presentation. In order to compare data from ultrasonic echo signals with data obtained from a reference workpiece or to compare a signal received at different times from the same workpiece, or to compare signals received from different locations in the workpiece, the cathode ray tube A-scans must be stored, for example, in a computer. The computer, in turn, may be preprogrammed for analyzing the data from the workpiece under test. Since the signal displayed on the screen is in analog form, it must first be converted into a digital form suitable for storage.
A common method presently employed for processing the analog A-scan data to derive a digital form is the "sample-and-hold" method which will be explained hereinafter in connection with the present invention, and which method is described in "Elektronik", 1975, Vol. II, pages 85-86 and Vol. III, pages 105-106, published by Fransis-Verlag, Munich. The sample-and-hold method requires a large number of sequential samples in order to obtain good resolution of the echo signal. Of course, the larger number of samples requires longer processing and storage time.
Accordingly, it is an object of this invention to provide a novel method and apparatus for converting analog ultrasonic echo signals into digital form exhibiting substantially faster response time without loss of resolution and without increasing the repetition rate of the ultrasonic test system employed.
A further object of this invention is to provide a new and improved method and apparatus for converting analog ultrasonic echo signals into digital form in a manner to save processing and storage time.